Interpretive Summary: Bluetongue viruses (BTV) are insect-transmitted orbiviruses of importance to the cattle and sheep industry. Viruses that were characterized as pathogenic were genetically compared to viruses that were not pathogenic. Thirty-one of thirty-four chances identify were found to be consistent within each characterized group. These findings may lead to genetic test to identify pathogenic strains of BTV.

Technical Abstract:
Bluetongue virus (BTV) is an insect-transmitted orbivirus of importance to the cattle and sheep industry. The VP2 protein, encoded by L2, contains neutralizing epitopes. Previously, a panel of neutralizing monoclonal antibodies (MAbs) to the BTV serotype 17 (BTV-17) prototype strain was generated and it was determined that the neutralization domain consists of three overlapping epitopes (Bernard et al., 1996). Over thirty amino acid changes were found between a neutralized BTV-17 prototype strain and a non-neutralized BTV-17 198 strain (Bernard et al., 1997). In this study, the L2 genes from eight additional strains, representing both the neutralized and non-neutralized groups of BTV-17, were sequenced to determine the degree of conservation of the previously characterized differences. Comparison of the deduced amino acid sequences showed that 91% (30/33) of the previously noted changes were conserved within each group. The sequence of the M5 gene that encodes VP5 was also examined, since this surface protein has also been shown to affect neutralization. No consistent changes were noted between the neutralized and non-neutralized groups of BTV-17 by analysis of the VP5 protein. Finally, the L2 sequences of five MAb neutralization escape mutants (Bernard et al., 1996) were determined to identify specific amino acids involved in neutralization and perhaps virulence. All five mutants contained 1-3 amino acid changes that were in close proximity to a previously described variable region. These changes are likely responsible for the overlapping neutralization sites described previously. This is the first description of two BT virus populations that have distinct neutralization characteristics co-circulating in a defined geographical region.